Silicon photonic elements rely on the principle of resonance to achieve active and passive optical functionality such as filtering, dense wavelength division multiplexing (DWDM), modulation, and switching. Realizing these functions in an efficient manner critically depends on spectral alignment of resonance and operating wavelengths. Use of resonators with high quality factors (e.g., Q factors exceeding 1000) makes silicon microphotonic elements sensitive to variations in temperature and fabrication processes. Unpredictable, real-time changes in silicon chip temperature cause the spectral resonance location to shift from its desired value, which may worsen the insertion loss, contrast ratio, passband ripple, rejection ratio and, ultimately, the signal-to-noise ratio at an associated photoreceiver. These temperature-induced changes in the optical response of resonant devices are a dynamic effect.
Besides these temperature-induced dynamic changes, small variations in the fabricated dimensions of resonant devices can also create significant deviations in resonance location from a predetermined value, and can also lead to worsening in the optical figures of merit described above. Fabrication-induced changes in optical response are a static effect (as opposed to the dynamic temperature-induced changes). Together, variations in temperature and fabrication lead to enough time-dependent shifts in the photonic system response to warrant real-time monitoring and correction of resonance locations.
A prevalent technique to bring a resonant wavelength back to its design specification value is referred to as “thermo-optic tuning”. It is known that the refractive index value of silicon—the base material used to form photonic resonators—is a function of temperature. Thus, by changing the temperature of the photonic resonator, its local refractive index will also change and the resonant wavelength will shift accordingly. Attractive features of thermo-optic tuning include its reversibility, unlimited number of tune-detune cycles, and allowance for tuning of individual resonators or resonator groups.
Thermo-optic tuning is typically provided by including a resistor-like element (as simple as a length of a metal conductor) in proximity to the optical resonator. A DC current is passed through the element, which generates heat by well-known effects. The amount of heat that is generated is a function of, among other things, the selected material composition of the conductive element, its topology, proximity to the resonator and value of DC current applied to the element.
In applications where two or more photonic resonators are used (common in many filtering and switching situations), each resonator is likely to be tuned to a different resonant wavelength. In this case, a separate heater element (and DC current source) is paired with each resonator. From a circuit design point of view, it is desirable to connect the various current sources to a common ground plane taking the form of a metallic connection (the ground line) between adjacent resonators. This common ground connection, however, creates an undesirable thermal path between adjacent resonators, making it difficult to tune closely-spaced resonators. That is, even if other means are being used to provide thermal isolation (such as, for example, physical separation between adjacent photonic resonators), the utilization of a common ground connection provides a thermal path to transfer unwanted (and unpredictable) heat from one resonator to another.
A need remains, therefore, for a way to maintain an electrical connection between adjacent photonic resonators while still keeping them thermally isolated so that individual resonant wavelength tuning can be performed.